There is provided a catalyst and a process for the direct partial oxidation of methane with oxygen, whereby hydrocarbons having at least two carbon atoms are produced. The catalyst used in this reaction comprises a spinel oxide, such as MgMn.sub.2 O.sub.4 or CaMn.sub.2 O.sub.4, modified with an alkali metal, such as lithium or sodium.
Natural gas is an abundant fossil fuel resource. Recent estimates places worldwide natural gas reserves at about 35.times.10.sup.14 standard cubic feet corresponding to the energy equivalent of about 637 billion barrels of oil.
The composition of natural gas at the wellhead varies but the major hydrocarbon present is methane. For example the methane content of natural gas may vary within the range of from about 40 to 95 vol %. Other constituents of natural gas may include ethane, propane, butanes, pentane (and heavier hydrocarbons), hydrogen sulfide, carbon dioxide, helium and nitrogen.
Natural gas is classified as dry or wet depending upon the amount of condensable hydrocarbons contained in it. Condensable hydrocarbons generally comprise C.sub.3 + hydrocarbons although some ethane may be included. Gas conditioning is required to alter the composition of wellhead gas, processing facilities usually being located in or near the production fields. Conventional processing of wellhead natural gas yields processed natural gas containing at least a major amount of methane.
Processed natural gas, consisting essentially of methane, (typically 85-95 volume percent) may be directly used as clean burning gaseous fuel for industrial heat and power plants, for production of electricity, and to fire kilns in the cement and steel industries. It is also useful as a chemicals feedstock, but large-scale use for this purpose is largely limited to conversion to synthesis gas which in turn is used for the manufacture of methanol and ammonia. It is notable that for the foregoing uses no significant refining is required except for those instances in which the wellhead-produced gas is sour, i.e., it contains excessive amounts of hydrogen sulfide. Natural gas, however, has essentially no value as a portable fuel at the present time. In liquid form, it has a density of 0.415 and a boiling point of minus 162.degree. C. Thus, it is not readily adaptable to transport as a liquid except for marine transport in very large tanks with a low surface to volume ratio, in which unique instance the cargo itself acts as refrigerant, and the volatilized methane serves as fuel to power the transport vessel. Large-scale use of natural gas often requires a sophisticated and extensive pipeline system.
A significant portion of the known natural gas reserves is associated with fields found in remote, difficulty accessible regions. For many of those remote fields, pipelining to bring the gas to potential users is not economically feasible.
Indirectly converting methane to methanol by steam-reforming to produce synthesis gas as a first step, followed by catalytic synthesis of methanol is a well-known process. The Mobil Oil Process, developed in the last decade provides an effective means for catalytically converting methanol to gasoline, e.g. as described in U.S. Pat. No. 3,894,107 to Butter et al. Although the market for gasoline is huge compared with the market for methanol, and although this process is currently used in New Zealand, it is complex and its viability appears to be limited to situations in which the cost for supplying an alternate source of gasoline is exceptionally high. There evidently remains a need for other ways to convert natural gas to higher valued and/or more readily transportable products.
One approach to utilizing the methane in natural gas is to convert it to higher hydrocarbons (e.g. C.sub.2 H.sub.6 ; C.sub.2 H.sub.4 ; C.sub.3 H.sub.8 ;C.sub.3 H.sub.6...); these have greater value for use in the manufacture of chemicals or liquid fuels. For example, conversion of methane to ethane or ethylene, followed by reaction over a zeolite catalyst can provide a route to gasoline production that entails fewer steps than the indirect route via methanol synthesis described above. Unfortunately, the thermal conversion of methane to ethane is a thermodynamically unfavorable process (.DELTA.G.degree..gtoreq.+8kcal/mol CH.sub.4) throughout the range from 300-1500K. The upgrading reactions explored here are oxidative conversions of methane to higher hydrocarbons, as exemplified in the following equations. EQU CH.sub.4 +0.25 O.sub.2 .fwdarw.0.5 C.sub.2 H.sub.6 +0.5 H.sub.2 O EQU CH.sub.4 +0.50 O.sub.2 .fwdarw.0.5 C.sub.2 H.sub.4 +1.5 H.sub.2 O
Analogous reactions include those converting methane to C.sub.3, C.sub.4,...and higher hydrocarbons. These oxidation processes have very favorable free energy changes (.DELTA.G.degree.&lt;-19 kcal/mol CH.sub.4) throughout the temperature range of 300-1000K. The oxidation reactions are commonly performed in the presence of a catalyst. The use of the catalyst allows the reaction to occur under conditions where there is essentially no thermal reaction between methane and oxygen. The catalyst can also favorably influence the selectivity of the oxidation reaction to minimize the extent of over-oxidation to CO and CO.sub.2.
While numerous catalysts have been employed in this reaction, there has not been any report regarding application of spinels as catalysts to upgrade methane/natural gas in this manner. The application described here likewise stands in contrast to references citing spinels as catalysts for total combustion (Happel, J.; Hnatow, M.; Bajars, L. "Base Metal Oxide Catalysts"; Marcel Dekker, Inc.: New York, N.Y., 1977) or as catalysts for oxidative chlorination of methane to methylchloride (Vlasenko, V.M., et al, Kinet. Katal, 1984, 22, 28).